Radio jamming

A radio jammer is any device that deliberately blocks, jams or interferes with authorized wireless communications.[1] In the United States, jammers are illegal and their use can result in large fines.[1]

The concept can be used in wireless data networks to disrupt information flow.[3] It is a common form of censorship in totalitarian countries, in order to prevent foreign radio stations in border areas from reaching the country.[2]

Jamming is usually distinguished from interference that can occur due to device malfunctions or other accidental circumstances. Devices that simply cause interference are regulated under different regulations. Unintentional 'jamming' occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable television plant that accidentally emits on an aircraft emergency frequency.

Originally the terms were used interchangeably but nowadays most radio users use the term "jamming" to describe the deliberate use of radio noise or signals in an attempt to disrupt communications (or prevent listening to broadcasts) whereas the term "interference" is used to describe unintentional forms of disruption (which are far more common). However the distinction is still not universally applied. For inadvertent disruptions, see electromagnetic compatibility.

Intentional communications jamming is usually aimed at radio signals to disrupt control of a battle. A transmitter, tuned to the same frequency as the opponents' receiving equipment and with the same type of modulation, can, with enough power, override any signal at the receiver. Digital wireless jamming for signals such as Bluetooth and WiFi is possible with very low power.

The most common types of this form of signal jamming are random noise, random pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse, spark, recorded sounds, gulls, and sweep-through. These can be divided into two groups – obvious and subtle.

Obvious jamming is easy to detect because it can be heard on the receiving equipment. It usually is some type of noise such as stepped tones (bagpipes), random-keyed code, pulses, music (often distorted), erratically warbling tones, highly distorted speech, random noise (hiss) and recorded sounds. Various combinations of these methods may be used often accompanied by regular morse identification signal to enable individual transmitters to be identified in order to assess their effectiveness. For example, China, which used jamming extensively and still does, plays a loop of traditional Chinese music while it is jamming channels (c.f. Attempted jamming of numbers stations).

The purpose of this type of jamming is to block reception of transmitted signals and to cause a nuisance to the receiving operator. One early Soviet attempt at jamming western broadcasters used the noise from the diesel generator that was powering the jamming transmitter.

Subtle jamming is jamming during which no sound is heard on the receiving equipment. The radio does not receive incoming signals yet everything seems superficially normal to the operator. These are often technical attacks on modern equipment, such as "squelch capture". Thanks to the FM capture effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier. The receiver locks onto the larger carrier signal and hence will ignore the FM signal with information.

Digital signals use complex modulation techniques such as QPSK. These signals are very robust in the presence of interfering signals. However, the signal relies on hand shaking between the transmitter and receiver to identify and determine security settings and method of high level transmission. If the jamming device sends initiation data packets the receiver will begin its state machine to establish two way data transmission. A jammer will loop back to the beginning instead of completing the handshake. This method jams the receiver in an infinite loop where it keeps trying to initiate a connection but never completes it, which effectively blocks all legitimate communication.

Bluetooth and other consumer radio protocols such as WiFi have built in detectors so that they only transmit when the channel is free. Simple continuous transmission on a given channel will continuously stop a transmitter transmitting, hence jamming the receiver from ever hearing from its intended transmitter. Other jammers work by analysing the packet headers and depending on the source or destination, selectively transmit over the end of the message, corrupting the packet.

During World War II, ground radio operators would attempt to mislead pilots by false instructions in their own language, in what was more precisely a spoofing attack than jamming. Radar jamming is also important to disrupt use of radar used to guide an enemy's missiles or aircraft. Modern secure communication techniques use such methods as spread spectrum modulation to resist the deleterious effects of jamming.

Jamming of foreign radio broadcast stations has often been used in wartime (and during periods of tense international relations) to prevent or deter citizens from listening to broadcasts from enemy countries. However such jamming is usually of limited effectiveness because the affected stations usually change frequencies, put on additional frequencies and/or increase transmission power.

In occupied Europe the Nazis attempted to jam broadcasts to the continent from the BBC and other allied stations. Along with increasing transmitter power and adding extra frequencies, attempts were made to counteract the jamming by dropping leaflets over cities instructing listeners to construct a directional loop aerial that would enable them to hear the stations through the jamming. In the Netherlands such aerials were nicknamed "moffenzeef" (English: "kraut sieve").[7]

During much of the Cold War, Soviet (and Eastern Bloc) jamming of some Western broadcasters led to a "power race" in which broadcasters and jammers alike repeatedly increased their transmission power, utilised highly directional antennas and added extra frequencies (known as "barrage" or "frequency diversity" broadcasting) to the already heavily overcrowded shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Soviet stations) suffered from the rising levels of noise and interference.[8][9]

Meanwhile, some listeners in the Soviet Union and Eastern Bloc devised ingenious methods (such as homemade directional loop antennas) to hear the Western stations through the noise. Because radio signal radio propagation on shortwave can be difficult to predict reliably listeners sometimes found that there were days/times when the jamming was particularly ineffective because radio fading (due to atmospheric conditions) was affecting the jamming signals but favouring the broadcasts (a phenomenon sometimes dubbed "twilight immunity"). On other days of course the reverse was the case. There were also times when jamming transmitters were (temporarily) off air due to breakdowns or maintenance. The Soviets (and most of their Eastern bloc allies) used two types of jamming transmitter. Skywave jamming covered a large area but for the reasons described was of limited effectiveness. Groundwave jamming was more effective but only over a small area and was thus only used in/near major cities throughout the Eastern Bloc. Both types of jamming were less effective on higher shortwave frequencies (above 15 MHz); however, many radios in the Soviet Union didn't cover the higher bands.[11] Skywave jamming was usually accompanied by morse signals in order to enable (coded) identification of the jamming station in order that Soviet monitoring posts could assess the effectiveness of each station.

In 1987 after decades of generally refusing to acknowledge that such jamming was even taking place the Soviets finally stopped jamming western broadcasts with the exception of RFE/RL which continued to be jammed for several months into 1988. Previously there had been periods when some individual Eastern bloc countries refrained from jamming Western broadcasts but this varied widely by time and country. In general outside of the Soviet Union itself Bulgaria was one of the most prolific operators of jamming transmitters in the Eastern bloc with East Germany the least.

While western governments may have occasionally considered jamming broadcasts from Eastern Bloc stations, it was generally accepted that doing so would be a pointless exercise. Ownership of shortwave radios was less common in western countries than in the Soviet Union where due to the vast physical size of the country many domestic stations were relayed on shortwave as it was the only practical way to cover remote areas. Additionally western governments were generally less afraid of intellectual competition from the Eastern Bloc.

In Francoist Spain the dictatorship jammed for decades Radio España Independiente, the radio station of the Communist Party of Spain which broadcast from Moscow (1941-1955) and Bucharest (1955-1977). It was the most important clandestine radio in Spain and the regime considered it a threat, since it allowed its citizens to skip the censorship of the local media.[12] Money and technological assistance for the jamming came from the United States.

However, in Latin America there were instances of communist radio stations such as Radio Venceremos being jammed, allegedly by the CIA, while there were short lived instances where Britain jammed some Egyptian (during the Suez Crisis), Greek (prior to Cyprus gaining independence) and Rhodesian stations.[13]

In 2002, China acquired standard short-wave radio-broadcasting equipment designed for general public radio-broadcasting and technical support from Thales Broadcast Multimedia, a former subsidiary of the French state-owned company Thales Group.

Adele Milna (BSEE) of Continental Electronics (in an audio file held at shortwave.org) claims that China has duplicated his company's 100 kW, 250 kW shortwave transmitters.[14] It is unclear if these products were indeed duplicated or if broadcast jamming (as opposed to future product sales) were a reason for the duplication.

Debates have been raised in Iran regarding the possible health hazards of satellite jamming. Iranian officials including the health minister have claimed that jamming has no health risk for humans. However, the minister of communication has recently admitted that satellite jamming has 'serious effects' and has called for identification of jamming stations so they can put a stop to this practice.[15][16][17] The government has generally denied any involvement in jamming and claimed they are sent from unknown sources.[18] According to some sources IRGC is the organization behind satellite jamming in Iran.[19]

The Russian armed forces have, since the summer of 2015, begun using a multi-functional EW weapon system in Ukraine, known as Borisoglebsk 2.[20] It is postulated that this system has defeated communications in parts of that country, including mobile telephony and GPS systems.[20][21][22]

Since the early 1960s, the practice of radio jamming has been very common in Cuba, blocking not only American government funded radio stations (such as VOA) but also radio stations owned and/or operated by (or selling airtime to) Cuban exile groups transmitting from Miami, such as La Cubanisima, Radio Mambi, WWFE La Poderosa and Cadena Azul.[citation needed] The same practice has been applied to Radio Martí and TV Martí, operated by the U.S. Information Agency since 1985.

Pakistan has recently stated its intention to begin jamming clandestine radio stations operated by the Taliban and Mobile operators signals in all jails across Pakistan. This decision has led to huge hue and cry by Pakistani cellular operators as most of the jails lie in middle of Urban areas and these have huge impact on cellular service of all operators in the adjacent area of jails.[25]